1. Field of the Invention
The present invention relates to a process for the control of an alternating current plasma panel, as well as to an apparatus for performing the same.
2. Description of the Prior Art
A plasma panel is a device making it possible to display characters, figures and curves, i.e. bidimensional images obtained by combining zones or "points" of a surface, which are made bright by appropriate controls. Such devices are well known to the Expert and are more particularly described in the article which was published in "Revue Technique THOMSON-CSF", June 1978, vol. 10, no. 2, pp. 249 to 275.
More specifically a plasma panel is a juxtapositioning of a large number of cells arranged in matrix-like manner, each cell comprising a gas-filled space, located at the intersection of two electrodes belonging to two orthogonal electrode systems and which are subject to control signals constituted by the difference between voltages applied to the two electrodes between which it is located.
In general, three types of control signals are used in plasma panels, namely entering or writing signals, which switch on the cells, erase signals which switch off the cells and maintenance signals which maintain the cells in their initial state, i.e. either the switched on or the switched off state.
However, unlike the maintenance signals applied to all the electrodes of the panel for ensuring the display of the entered information, the writing and erase signals are selective signals which may only bring about writing and erasure for the selected cells. Thus, a random cell xy is only written or erased if its two electrodes x and y receive adequate voltages Vx and Vy making it possible to obtain the writing or erase signal at the terminals of this single cell.
Therefore, the control electronics must incorporate circuits making it possible to selectively apply to the electrodes the voltages necessary for the operation of the panel. Various alternating plasma panel control circuits have been described in the prior art and particular reference is made to the article published by Texas Instruments, in November 1980 Bulletin SCA-204 and entitled "A.C. Plasma Display", as well as French Patent Application No. 81 19941 in the name of THOMSON-CSF, and corresponding U.S. patent application Ser. No. 431,152, which disclosed integrated circuits making it possible to control A.C. plasma panels.
These integrated circuits essentially comprise a low voltage logic part defining the signal to be effected, its duration and the electrodes of the panel to which the signal is applied, as well as a low voltage-high voltage interface which is controlled by the logic part, said interface making it possible to apply, to the electrodes of the panels, signals having a variable amplitude and duration as a function of the instruction to be carried out.
The logic part essentially comprises series-parallel shift registers and a decoding and validation system. Therefore, the logic addresses or data designating the active and inactive electrodes are entered in series into the shift registers and are then obtained in parallel at the outputs of the registers, which respectively correspond to the electrodes of the plasma panel. An instruction defining the writing or erase signal to be applied to the active electrodes then validates the parallel outputs of the registers towards the low voltage (LV)-high voltage (HV) interface.
In order to permit the easy display of a text or graphic symbol, the most complicated plasma panel control circuits must permit operation in two modes, namely a superimposing mode permitting the writing or erasure of one or more points without modifying the other points and a replacement mode making it possible to replace the information displayed on a segment or part of a vertical or horizontal segment by new information.
However, in order to carry out an information replacement, the solution which is conventionally used consists of supplying an erase instruction to all the points of the segment or of the part of the segment which is to be modified, followed by the entry or writing in of the points to be switched on. This control process suffers from a certain number of disadvantages. It takes a long time, because it is firstly necessary to reenter the address of all the electrodes of a segment to be erased and then the address of only those electrodes which are to be entered. In addition, the writing of a point immediately after its erasure causes problems with respect to the stabilization of the charges at the cell terminals.